pseudocode for char-building loop

replace unmanageable brute-force string building with loop that bit-shifts, stores byte, masks/shifts proper 4 bits, 
calls subroutine to get index into string and index into lookup table (current pseudocode has index-calculation discreetly done), and in subroutine, move byte from lookup part of string to string part of string.

will be easier to debug if char is being copied to beginning end of string